a. Field of the Invention
The invention relates to an illumination module and, more particularly, to an illumination module having a color temperature conversion medium.
b. Description of the Related Art
Compared with a traditional light source, a light-emitting diode (LED) has the advantages of long lifetime, high luminous efficiency, radiation free, and low power consumption. Therefore, recent trends in the design of an illumination lamp are towards replacing the traditional light source with light-emitting diodes. Generally, the emission color of an illuminating apparatus is determined by the color temperature of a selected LED. Typically, the color temperature of a common illuminating lamp is in the range of 2700K and 7000K. However, light-emitting diodes for illuminating purposes conforming to such range of color temperature are too expensive compared with a traditional light source. Thus, the market share of LEDs in the illuminating applications is relatively low. Typically, LEDs for display purposes are usually much cheaper than LEDs for illuminating purposes, but LEDs for display purposes usually have a color temperature of larger than 9000K to fail to serve illuminating purposes. Therefore, if LEDs used in a display can be modified to fit an illuminating lamp, the fabrication costs of an illuminating lamp using LEDs as a light source are greatly reduced.
As shown in FIG. 1, US patent publication no. 20060072339 discloses a backlight module 100 having a light-emitting diode 102, a light guide plate 104 and a phosphor 106. The light guide plate 104 is disposed adjacent to the light-emitting diode 102, and the phosphor 106 is disposed between the light-emitting diode 102 and the light guide plate 104. The light-emitting diode 102 is, for example, a blue light-emitting diode or a blue light laser diode, and the phosphor 106 includes a yellow fluorescent material. When the phosphor 106 is excited by the blue light emitted by the blue light-emitting diode or the blue light laser diode, the phosphor 106 may emit yellow light. Then, the yellow light is mixed with the blue light emitted by the blue light-emitting diode or the blue light laser diode in the light guide plate 104 to generate white light. Finally, the white light is outputted from the light-emitting surface of the light guide plate 104. Besides, Taiwan patent publication no. 201033542 discloses a light-emitting device having a light-emitting diode, a light guide plate, and a phosphor-containing plate. The phosphor-containing plate may be disposed on a light-emitting surface, a bottom surface, or a light incident surface of the light guide plate. When a light beam of the light-emitting diode passes through the phosphor-containing plate, part of the light beam is transformed into a different color and then mixed with the untransformed light beam to generate white light.
In the above conventional designs, the excitation of fluorescent materials is merely applied to a backlight module to form white light having a high color temperature. However, such high-color-temperature white light fails to be used in a common illumination lamp. In other prior design, U.S. Pat. No. 6,345,899 discloses a backlight module, where yellow and blue ink droplets are distributed on a surface of a reflective plate to compensate for variations in light reflectivity as a result of different wavelengths of light on the reflective plate and to compensate for color non-uniformity of the backlight module as a result of different scattering rates of the light guide plate. Taiwan patent publication no. 200700834 discloses a backlight module, where an ink layer having color-shift particles is formed on a lower surface of a light guide plate to absorb red and green light to compensate for color shift. However, the yellowish color shift in the front end of the light guide plate to cause color non-uniformity of the backlight module still remains.